An ink jet system utilizing heat energy disclosed in U.S. Pat. No. 4,723,129, U.S. Pat. No. 4,740,796, etc. can provide high speed, high density and high definition recording of a high quality and is suitable for color recording and also for compact designing. Accordingly, progressively increasing attention has been paid to such ink jet system in recent years. In a representative one of apparatus which employ such system, ink as the recording liquid is discharged utilizing heat energy, and accordingly, it has a heat acting portion which causes heat to act upon the ink. In particular, a heat generating resistor is provided for an ink pathway, and making use of heat energy generated from the heat generating resistor, ink is heated suddenly to produce an air bubble by which the ink is discharged.
The heat acting portion, in view of causing heat to act upon an object, a portion apparently similar in construction to a conventional so-called thermal head. However, the heat acting portion is quite different in fundamental technology from the thermal head in such portions that it contacts directly with ink, that it is subjected to mechanical shock which is caused by cavitations produced by repetitions of production and extinction of bubbles of ink, or in some cases, further to erosion, that it is subjected to a rise and a drop of temperature over almost 1,000.degree. C. for a very short period of time of the order of 10.sup.-1 to 10 microseconds, and so forth. Accordingly, the thermal head technology cannot naturally be applied to the ink jet technology as it is. In other words, the thermal head technology and the ink jet technology cannot be discussed on the same level.
Incidentally, as for the heat acting portion of an ink jet head, since it is subjected to such severe environment as above described, it is a common practice to employ such a structure that an electric insulating layer made of, for example, SiO.sub.2, SiC, Si.sub.3 N.sub.4 or the like is disposed as a protective film on a heat generating resistor and a cavitation resisting layer made of Ta or the like is disposed thereon in order to protect the heat acting portion from environment in which it is used. As the constituent material of such protective layer for use with an ink jet head, such materials which are tough against a shock and erosion by a cavitation as described, for example, in U.S. Pat. No. 4,335,389 can be mentioned.
Apart from this, it is desired for the heat acting portion of an ink jet head to be designed such that heat generated from the heat generating resistor acts upon ink as efficiently and quickly as possible in order to save power consumption and improve the responsibility to a signal inputted. For this, other than the above-mentioned configuration in which the protective layer is provided, a configuration in which a heat generating resistor is disposed so as to directly contact with ink has been proposed by Japanese Patent Laid-open No. 126462/1980.
The ink jet head of this configuration is superior to the configuration in which the protective layer is provided with regard to thermal efficiency. However in this case, the heat generating resistor is subjected to a shock and erosion by cavitation and further to a rise and drop of temperature and in addition, to an electrochemical reaction which is caused by electric current which flows through recording liquid because the recording liquid contacts with heat generating resistor and has a conductivity. There are known various metals, alloys and metallic compounds, and cermets, beginning with Ta.sub.2 N and RuO.sub.2, as the constituent materials of heat generating resistors. However these are not always satisfactory in durability and stability when they are used as the constituent material of the heat generating resistor of the ink jet head of this configuration.
Some of ink jet heads of the configuration in which a protective layer is disposed as above described which have been proposed can be adopted in practical use in view of durability and resistance variation. However, it is very difficult, in any case, to perfectly prevent occurrence of defects which may take place at the time of forming the protective layer. This is a serious factor of reducing the yield in mass production. In recent years, there has been an increased demand for a further improvement in speed and density in recording. There is a tendency that the number of discharging outlets of an ink jet head is increased in order to cope with such demand. In this case, the above situation entails a serious problem.
Further, while the foregoing protective layer decreases the efficiency in transfer of heat from the heat generating resistor to the recording liquid, if the transfer efficiency is low, the entire power consumption required increases and a variation in temperature in the ink jet head upon driving increases. Such temperature variation results in causing a variation in volume of a liquid droplet discharged from a discharging outlet, which causes a variation in density of an image recorded. Meanwhile, if the number of discharging operations per unit time is increased in order to cope with an increase in recording speed, the power consumption by the ink jet head is heightened accordingly and as a result, the temperature variation is increased. Such temperature variation will bring about a corresponding density variation of an image obtained. Other than this, in the case of making an increase in the number of discharging outlets which involves an increase in density of electrothermal converting bodies, the power consumption by the ink jet head is heightened and as a result, the temperature variation is increased, resulting in making the resulting record images to have a variation in density corresponding such temperature variation. These problems of making the resulting record images to be varied in density are contrary to the demand for providing a high quality record image, and they are required to be solved as early as possible.
In order to solve these problems, early provision of an improved ink jet head of the configuration in which an heat generating resistor contacts directly with ink and which excels in the thermal efficiency is earnestly desired.
However, as already described in the above, in the conventional configuration in which ink contacts directly with the heat generating resistor, the heat generating resistor is subjected to expose to not only a shock or erosion by cavitation and further to a rise and drop of temperature but also to an electrochemical reaction. Because of this, the heat generating body constituted by a conventional material such as Ta.sub.2 N, RuO.sub.2, HfB.sub.2, or the like causes problems in durability such that it is meachanically destroyed, corroded or dissolved.
The materials which are disclosed in U.S. Pat. No. 4,335,389 as being tough against a shock or erosion by cavitation are understood to exhibit their effects for the first time when they are used as the constituent of such a protective layer (a cavitation resisting layer) as above described. However, in the case where any of these materials is employed for the heat generating resistor which contacts directly with ink, it is often dissolved or corroded by an electrochemical reaction, and because of this, a sufficient durability cannot be insured therefor.
In order to perform recording of a high definition and a high quality, stable ink discharging is essential. For this purpose, the heat generating resistor is necessary to be small in resistance variation. Incidentally, Ta or Ta--Al alloy described in Japanese Patent Laid-open No. 96971/1984 is comparatively superior, in the case where it is used as the constituent of the heat generating resistor of an ink jet head in which the heat generating resistor contacts directly with ink, in durability, particularly, in cavitation resisting property in that the heat generating resistor is not broken. However, in regard to a variation in resistance during the repetition of production of bubbles, any of Ta and Ta--Al alloy is not satisfactory in that the resistance variation is not small enough as desired. Further, any of Ta and Ta--Al alloy does not have a very high ratio M between an applied pulse voltage (V.sub.break) at which the heat generating resistor is broken and a bubble producing threshold voltage (V.sub.th) and does not have a very high heat resisting property. Consequently, they have a problem such that the lifetime of the heat generating resistor constituted by any of them is often greatly deteriorated even by a small increase in driving voltage (V.sub.op). In particular, any of Ta and Ta--Al is not always sufficiently high in resisting property to an electrochemical reaction. Because of this, when any of them is used as the constituent of the heat generating body of an ink jet head in which the heat generating body contacts directly with ink, if production of bubbles is repeated by a number of pulses applied, the electric resistance of the heat generating resistor is varied to a great extent. Thus, there is a problem in that the state of producing bubbles is also varied depending upon such variation in the electric resistance of the heat generating resistor. In addition, there is also a problem in that a small variation in the V.sub.op causes a significant influence on the lifetime of the heat generating resistor since the heat resisting property is not high enough as desired.
Thus, it is understood that in the case where the heat generating resistor which contacts directly with recording liquid (that is, ink) is formed of any of the known materials, there cannot be readily obtain an ink jet head or an ink jet apparatus which satisfies all of resistance to shock by cavitation, resistance to erosion by cavitation, mechanical durability, chemical stability, electrochemical stability, inherent resistance stability, heat resistance, oxidation resistance, dissolution resistance and resistance to thermal shock. Particularly, there cannot be obtained an ink jet head having the configuration in which the heat generating body is disposed so as to directly contact with ink and which is high in heat transfer efficiency, superior in signal responsibility, and has a satisfactory durability and a satisfactory liquid discharging stability.
The present inventors previously had accomplished inventions capable of solving those technical problems as above described (see, International Publication WO90/09888 (hereinafter referred to as Literature 1) and International Publication WO90/09887 (hereinafter referred to as Literature 2)). Particularly, by Literatures 1 and 2, the present inventors proposed the use of a Ir--Ta alloy containing these elements respectively at a specific composition rate and a Ir--Ta--Al alloy containing these elements respectively at a specific composition rate as the constituent of the heat generating resistor of an ink jet head. These alloys are ones which can satisfy, to a certain extent, all of resistance to shock by cavitation, resistance to erosion by cavitation, mechanical durability, chemical stability, electrochemical stability, inherent resistance stability, heat resistance, oxidation resistance, dissolution resistance and resistance to thermal shock. In these alloys, Ir is a material which is liable to exhibit superiority in terms of heat resistance, oxidation resistance and chemical stability.
Incidentally, in recent years, there is a tendency that the ink jet apparatus is miniaturized. In fact, a number of miniature ink jet apparatus having a secondary battery installed therein are commercially available.
By the way, commercially available ordinary second batteries are of a voltage of about 10 V. Ink jet apparatus having such secondary battery installed therein are usually used in such a way that a prescribed converter is installed and using this converter, the voltage of about 10 V of the secondary battery is raised to a doubled voltage of about 20 V. The reason for this is for attaining high speed recording at a high speed drive by shortening the drive pulse duration (the driving duration in other words).
As for these ink jet apparatus, there is an increased demand for further miniaturizing them. In order to cope with such demand, it is desired to eliminate the use of a converter. In the case of an ink jet apparatus with no converter, the situation comes to a result that a voltage of about 10 V of a second battery is used as the driving voltage, wherein the drive pulse duration (the driving duration) is necessary to be enlarged to an extent that ink can be discharged in a desired state, because the driving voltage is low. However, in the case of driving the ink jet apparatus with such relatively long drive pulse duration, any of the heat generating resistors described in Literatures 1 and 2 is not satisfactory especially in terms of durability. Consequently, there is a demand for early provision of a heat generating resistor which is sufficiently durable even in the case of driving the ink jet apparatus with a relatively long drive pulse duration.